1. Field of the Invention
This invention concerns the production of glass preforms for optical fibers, particularly preforms of the rod-in-tube (RIT) variety.
2. Discussion of the Known Art
Initial stages of RIT fiber preform manufacture include inserting one or more core rods axially inside of a glass overclad tube. Typically, a core rod has a handle extending from one end so that it can be transported manually without risk of contaminating the rod's core material. The rod handle may also be used for product identification and tracking purposes.
In conventional RIT processes, a single core rod is inserted in an axial passage in the overclad tube with the aid of the rod's attached handle. A sacrificial glass spacer may also be inserted to fill the entire length of the axial passage in the tube. The handle remains attached to the core rod until a final overcladding process step, at which time the handle is removed.
It is also known to use multiple core rods by welding them end to end to form a continuous length of core material. This is a costly additional step, however, and may have a negative effect on the quality of fiber drawn from the preform at locations corresponding to the welds, due to an added hydroxyl (OH) concentration induced by the weld heat source.
Process improvements such as placing multiple core rods inside a single overclad tube or the use of a mechanical assembly, require that the handle of the core rod be removed before inserting the core material of the rod inside one or more overclad tubes. For additional cost reduction and to maintain zero water peak quality in optical fibers such as, for example, AllWave® fibers available from OFS Fitel USA, the core rods must be deeply etched with HF acid to remove residual surface water from the VAD process before the rods are inserted in an overclad tube.
A technique that allows core rods without handles to be cleaned, etched, and inserted in an overclad tube without having to contact the rod material directly by hand, is disclosed in co-pending U.S. patent application Ser. No. 11/088,076 filed Mar. 23, 2005, entitled “Optical Fiber Preform with Overclad Tubes” and assigned to the assignee of the present application. In the disclosed technique, a glass spacer, a number of core rod segments, and a tube plug are loaded into an elongated tubular holder or sleeve. Plastics ball spacers are disposed between confronting end faces of the segments, between the glass spacer and an uppermost rod segment, and between the plug and a lowermost rod segment. The technique allows the glass spacer, the rod segments and the tube plug to be washed clean by flowing an acid solution between open front and rear ends of the tubular holder, followed by a rinse with deionized water. Once cleaned, an open front end of the holder is aligned with the axial passage in an overclad tube, and the core rod segments are urged successively into the tube passage by inserting a push rod through the open rear end of the holder.
A procedure that enables many core rods (or rod segments) to be cleaned simultaneously while loaded in multiple holders, would therefore significantly reduce the time and cost required to manufacture RIT preforms in volume.